1.College of Information Engineering, Quzhou College of Technology, Quzhou 324000, China 2.College of Physics and Electronic Information, Gannan Normal University, Ganzhou 341000, China 3.Key Laboratory of Optoelectronics Technology, Ministry of Education, Beijing University of Technology, Beijing 100124, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61575008, 61650404), the Natural Science Foundation of Jiangxi Province, China (Grant No. 20171BAB202037), the Technology Project of Jiangxi Provincial Education Department, China (Grant No. GJJ170819), and the Quzhou Science and Technology Project, China (Grant No. 2019K20)
Received Date:22 July 2020
Accepted Date:17 September 2020
Available Online:09 January 2021
Published Online:20 January 2021
Abstract:The metamaterial perfect absorber (MPA) is a new type of electromagnetic wave absorber first proposed by Landy. Compared with the traditional electromagnetic wave absorber, MPA has many advantages, including ultra-high absorption efficiency, ultra-thin, compact structure, easily tunable resonances, etc. so it is gradually applied to ultra-sensitive sensing, imaging, detection and other fields. Nowadays, the MPA research focuses on two areas. One area focuses on the absorption efficiency modulation and absorption wavelength tuning, and the other area is to broaden the absorption bandwidth and achieve high absorptions at different optical frequencies. Previously, the MPA absorption efficiency modulation or absorption wavelength tuning was realized by changing the device structure or the surrounding medium material. But these methods can increase the difficulty in processing and increase the device volume. In order to achieve the control of absorption wavelength and absorption efficiency without increasing the difficulty in processing or the device volume. We propose to use vanadium dioxide and graphene as the materials of MPA, which has high absorption efficiency in the infrared band. It is found that the absorption efficiency of MPA at 9.66 μm wavelength can reach 96% when the temperature of vanadium dioxide is 5 ℃ by using finite difference time domain (FDTD) method. However, when the vanadium dioxide temperature rises to 68 ℃, the absorption efficiency of MPA suddenly drops to 2.8%. The modulation depth of absorption efficiency can reach 97.08%. We propose that the MPA be able to control not only the absorption efficiency, but also the absorption wavelength. By changing the voltage of graphene, the chemical potential Ef of graphene can be controlled and the absorption wavelength of MPA can be tuned. When Ef increases from 0.1 eV to 3 eV, the absorption wavelength of MPA will be blue-shifted from 9.66 μm to 6.46 μm. The magnetic field distribution of MPA at the absorption wavelength shows that the MPA has a high absorption efficiency because of the Fabry-Pérot (FP) cavity resonance is formed in MPA. Therefore, the change of structure parameters of MPA will affect its absorption characteristics. It is found by the FDTD method that the absorption wavelength of MPA will be redshifted, when the radius, thickness, period and thickness of the nanocolumn array increase. This study can provide theoretical guidance for designing and preparing the controllable MPA, which has compact structure and low process difficulty merits. Keywords:metamaterial/ absorber/ vanadium dioxide/ graphene
利用FDTD方法首先研究了VO2处在相变前后MPA的吸收效率, 具体如图2所示, 在利用FDTD方法模拟计算时, 先建立一个超材料单元, 然后在x, y方向添加周期边界(periodic), 在z方向添加完美匹配层边界条件(perfectly matched layer, PML), 网格类型选用auto non-uniform形式, 精度设为5. 从图2中可以发现, 温度TV = 5 ℃时, 以VO2为材料的MPA在波长λ = 9.66 μm时的吸收效率可达96%, 而当TV达到VO2的相变温度68 ℃时, MPA的吸收效率仅有2.8%. VO2相变前后的吸收效率调制深度(ΔA = (Amax–Amin)/Amax)可达97.08%, 相比于文献[3,14,15]有了较大的提升. 在VO2温度从5 ℃升到68 ℃过程中, 其温度会传导给石墨烯, 但石墨烯的电导率并不会产生明显变化[29]. 此时, P = 0.8 μm, h = 0.05 μm, r = 0.38 μm, H = 0.3 μm, t = 0.05 μm, Ef = 0.1 eV, 入射光为TM偏振. 由图2可知通过改变VO2的温度, 成功实现了MPA吸收效率的控制. 本文中通过外部热源实现对VO2的温度调制, 石墨烯的化学势Ef受外部电压Vg控制, 因此VO2的温度石墨烯电导率影响较小, 另外根据文献[30]可知, 利用电压将石墨烯化学势从0.1 eV调制到3.0 eV, 对二氧化钒影响也较小, 所以可知石墨烯和VO2的两种调制方式互不干扰. 图 2 VO2不同温度下MPA的吸收效率 Figure2. Absorption efficiency of MPA at different temperature of VO2.
为了探寻MPA在VO2相变前后吸收效率能实现高调制深度的原因, 我们计算了MPA在TV分别为5 ℃和68 ℃时的磁场分布(入射波长λ = 9.66 μm), 具体如图3所示. 图3(a)是TV = 5 ℃时, MPA的磁场分布, 可以看到大部分能量都聚集在Al2O3和VO2上, 这说明入射光大部分被MPA所吸收. 另外从图3(a)可以知道, 在VO2相变前MPA之所以有高吸收效率, 是因为MPA整体结构形成了法布里-帕罗干涉 (Fabry-Pérot, FP)腔共振, 即入射光在Au衬底-Al2O3/VO2-石墨烯/Au纳米阵列之间形成了干涉增强吸收. 图 3 VO2不同温度下MPA磁场分布 (a) TV = 5 ℃; (b) TV = 68 ℃ Figure3. Magnetic field distribution of MPA at different temperature of VO2: (a) TV = 5 ℃; (b) TV = 68 ℃.
图3(b)是TV = 68 ℃时MPA的磁场分布, 可以发现因为温度升高, 使VO2产生了相变, 变成了具有金属特性的金属层, 所以当入射光照射到MPA后, 入射光的大部分能量被反射到空气当中, 而在Al2O3和VO2上的能量非常稀少, 从而导致MPA的吸收效率下降到2.8%. 在VO2相变温度68 ℃以下时, MPA能形成FP腔共振吸收, 实现高吸收效率, 是因为在达到相变温度前, VO2都是保持高透射的半导体状态. 而且在相变前VO2的折射率n和消光系数k随温度变化幅度都比较小, 当TV从5 ℃增加到40 ℃过程中, n只增加了0.5左右, 而k只增加了0.05左右, 具体如图4所示. 但是当TV突然增加到了相变温度68 ℃时, VO2的k相比于TV = 5 ℃时突然增加了0.3左右, n增加了6左右, 这就明显变成了高反射的金属状态[31], 所以当TV达到相变温度时, MPA会将入射光几乎全部反射到空气当中, MPA的吸收效率显著下降. 图4中VO2在不同温度下的折射率和消光系数由Material Studio获得, 它是一种具有多种先进算法的的综合性模拟工具, 可对晶体材料的性质进行模拟分析. 图 4 温度对VO2折射率和消光系数的影响 (a) 折射率n; (b)消光系数k Figure4. The influence of temperature on the refractive index and the extinction coefficient of VO2: (a) Refractive index n; (b) extinction coefficient k.
23.2.MPA吸收波长的控制 -->
3.2.MPA吸收波长的控制
利用FDTD方法研究了石墨烯化学势Ef对MPA吸收波长的影响, 具体结果如图5所示. 可以看到, 当Ef = 0.1 eV时, MPA的吸收波长为9.66 μm, 而当Ef = 3 eV时, MPA的吸收波长为6.46 μm, 吸收波长蓝移了3.2 μm. 因此利用石墨烯实现了对MPA吸收波长的控制. 此时, P = 0.8 μm, h = 0.05 μm, r = 0.38 μm, H = 0.3 μm, t = 0.05 μm, TV = 5 ℃, 入射光为TM偏振. 另外从图5中可以发现, 当Ef = 1 eV时, MPA吸收率最高, 可达99.1%. 图 5 石墨烯化学势对MPA吸收波长的影响 Figure5. The effect of graphene chemical potential on the absorption wavelength of MPA.
随着Ef增加, MPA吸收波长之所以会产生蓝移现象, 是因为石墨烯共振波长λre与石墨烯等效折射率有关, 具体关系为λre = α + β·neff, 其中α, β是与结构参数、周围介电常数有关的常数[32]. 根据(2)式、(6)式和图6可知, 当Ef增加时, neff会逐渐下降, 从而导致石墨烯共振波长λre逐渐减小, 进而影响了MPA的吸收波长[19,31]. 根据(3)式—(5)式可知, 对于石墨烯Ef的控制, 只需通过改变外接电压Vg就可实现MPA吸收波长的控制. 图 6 石墨烯化学势对石墨烯等效折射率的影响 Figure6. The effect of graphene chemical potential on the equivalent refractive index of graphene.